koreader as default for every epub except those from .acsm

[EastEriq]

I like koreader, so my /mnt/ext1/system/config/extensions.cfg is very partial to it. However I also borrow DRM books from libraries, so I get .acsm. When I tap the acsm file icon on the home screen, or the entry in the librarian, the content is downloaded, the acsm file is deleted and the reader goes on opening the resulting epub. As expected, but since extensions.cfg says

epub:@EPUB_file:1:koreader.app,eink-reader.app:ICON_EPUB

then koreader is immediately called up for opening the book, and it complains that it cannot read the encrypted content. Also this is expected. So I have to close the book, go to the librarian, search for the book again, long press on it, and choose Open with PB Reader. This is firmware 5.2, long press does not even offer the choice on the home screen, and it's a bit of a PITA. True that then the preferred program for opening the book is remembered (stored in handlers.cfg), but still a nuisance.

Is anyone aware of any trick I could play for triggering pbreader only on epubs coming from acsm downloads, while leaving koreader as preferred default for all others?